I have been using the At&T 3G for connectivity on my N810 - and while sometimes it seems slow (not anywhere as slow as EDGE (which I had tried before with a EDGE phone) it is fast for most purposes and most days.

The extra network traffic due to all the 3G iPhone users may be the reason for latency issues sometimes, but I dont think it has downgraded the bandwidth in any drastic way.

Edit : Nowadays I use the same AT&T 3G network from Baltimore (where I work these days) and certainly in Baltimore my 3G works noticably faster and than in New York and faster for most of the time (so you may have a point there again).